The present application claims priority to Japanese Application(s) No(s). P2001-161890 filed May 30, 2001, which application(s) is/are incorporated herein by reference to the extent permitted by law.
The present invention relates to an active matrix type display apparatus having an active device in each pixel and controlling display in the pixel unit by means of the active device, and a driving method thereof, and particularly to an active matrix type display apparatus using an electrooptic device that varies brightness according to a current flowing therein, an active matrix type organic EL display apparatus using an organic-material electroluminescence (hereinafter described as organic EL (electroluminescence)) device as the electrooptic device, and driving methods thereof.
A liquid crystal display using a liquid crystal cell as a display device of a pixel, for example, has a large number of pixels arranged in a matrix manner, and controls light intensity in each pixel according to information of an image to be displayed, thereby effecting driving for image display. The same display driving is effected by an organic EL display using an organic EL device as a display device of a pixel and the like.
Since the organic EL display is a so-called self-luminous type display using a light emitting device as a display device of a pixel, however, the organic EL display has advantages such as higher visibility of images, no need for a backlight, and a higher response speed as compared with the liquid crystal display. Moreover, brightness of each light emitting device is controlled by the value of a current flowing therein. That is, the organic EL display differs greatly from the liquid crystal display or the like of a voltage-controlled type, in that the organic EL device is of a current-controlled type.
As with the liquid crystal display, the organic EL display uses a passive matrix method and an active matrix method as its driving method. Although the former has a simple construction, however, the former has problems such as difficulty in realizing a large high-definition display. Thus, the active matrix method has recently been actively developed which controls a current flowing through a light emitting device within a pixel by means of an active device, for example an insulated gate field-effect transistor (typically a thin film transistor; TFT) also disposed within the pixel.
FIG. 1 shows a conventional example of a pixel circuit (circuit of a unit pixel) in an active matrix type organic EL display (for more detailed description, see U.S. Pat. No. 5,684,365 and Japanese Patent Laid-Open No. Hei 8-234683).
As is clear from FIG. 1, the pixel circuit according to the conventional example includes: an organic EL device 101 having an anode connected to a positive power supply Vdd; a TFT 102 having a drain connected to a cathode of the organic EL device 101 and a source connected to a ground (hereinafter described as xe2x80x9cgroundedxe2x80x9d); a capacitor 103 connected between a gate of the TFT 102 and the ground; and a TFT 104 having a drain connected to the gate of the TFT 102, a source connected to a data line 106, and a gate connected to a scanning line 105.
Since the organic EL device has a rectifying property in many cases, the organic EL device may be referred to as an OLED (Organic Light Emitting Diode). Therefore, in FIG. 1 and other figures, a symbol of a diode is used to denote the organic EL device as the OLED. In the following description, however, a rectifying property is not necessarily required of the OLED.
The operation of the thus formed pixel circuit is as follows. First, when potential of the scanning line 105 is brought to a selected state (high level in this case) and a writing potential Vw is applied to the data line 106, the TFT 104 conducts, the capacitor 103 is charged or discharged, and thus a gate potential of the TFT 102 becomes the writing potential Vw. Next, when the potential of the scanning line 105 is brought to a non-selected state (low level in this case), the TFT 102 is electrically disconnected from the scanning line 105, while the gate potential of the TFT 102 is stably retained by the capacitor 103.
A current flowing through the TFT 102 and the OLED 101 assumes a value corresponding to a gate-to-source voltage Vgs of the TFT 102, and the OLED 101 continues emitting light at a brightness corresponding to the value of the current. The operation of selecting the scanning line 105 and transmitting to the inside of the pixel brightness data supplied to the data line 106 will hereinafter be referred to as xe2x80x9cwriting.xe2x80x9d As described above, once the pixel circuit shown in FIG. 1 writes the potential Vw, the OLED 101 continues emitting light at a fixed brightness until next writing.
An active matrix type display apparatus (organic EL display) can be formed by arranging a large number of such pixel circuits (which may hereinafter be described simply as pixels) 111 in a matrix manner as shown in FIG. 2, and repeating writing from a voltage driving type data line driving circuit (voltage driver) 114 through data lines 115-1 to 115-m while selecting scanning lines 112-1 to 112-n sequentially by a scanning line driving circuit 113. A pixel arrangement of m columns and n rows is shown in this case. Of course, in this case, the number of data lines is m and the number of scanning lines is n.
Each light emitting device in a passive matrix type display apparatus emits light only at an instant when the light emitting device is selected, whereas a light emitting device in an active matrix type display apparatus continues emitting light even after completion of writing. Thus, the active matrix type display apparatus is advantageous especially for use as a large high-definition display in that the active matrix type display apparatus can decrease peak brightness and peak current of the light emitting device as compared with the passive matrix type display apparatus.
In an active matrix type organic EL display, a TFT (thin film field-effect transistor) formed on a glass substrate is generally used as an active device. It is known, however, that amorphous silicon and polysilicon used to form the TFT have inferior crystallinity and inferior controllability of the conducting mechanism to single-crystal silicon, and thus the formed TFT has great variations in characteristics.
When a polysilicon TFT is formed on a relatively large glass substrate, in particular, the polysilicon TFT is generally crystallized by a laser annealing method after formation of an amorphous silicon film, in order to avoid problems such as thermal deformation of the glass substrate. However, it is difficult to irradiate the large glass substrate with uniform laser energy, and thus the crystallized state of the polysilicon is varied depending on a location within the substrate. As a result, the threshold value Vth of even TFTs formed on the same substrate can be varied from pixel to pixel by a few hundred mV, or 1 V or more in some cases.
In that case, even when the same potential Vw is written to different pixels, for example, the threshold value Vth of the TFTs varies from pixel to pixel. This results in great variation from pixel to pixel in the current Ids flowing through the OLED (organic EL device), and hence deviation of the current Ids from a desired value. Therefore high picture quality cannot be expected of the display. This is true for not only variation in the threshold value Vth but also variation in carrier mobility xcexc and the like.
In order to remedy such a problem, the present inventor has proposed a current writing type pixel circuit shown in FIG. 3 as an example (see International Publication Number WO01/06484).
As is clear from FIG. 3, the current writing type pixel circuit includes: an OLED 121 having an anode connected to a positive power supply Vdd; an N-channel TFT 122 having a drain connected to a cathode of the OLED 121 and a source grounded; a capacitor 123 connected between a gate of the TFT 122 and the ground; a P-channel TFT 124 having a drain connected to a data line 128, and a gate connected to a scanning line 127; an N-channel TFT 125 having a drain connected to a source of the TFT 124, and a source grounded; and a P-channel TFT 126 having a drain connected to the drain of the TFT 125, a source connected to the gate of the TFT 122, and a gate connected to the scanning line 127.
The thus formed pixel circuit is crucially different from the pixel circuit shown in FIG. 1 in the following respect: in the case of the pixel circuit shown in FIG. 1, brightness data is supplied to the pixel in the form of voltage, whereas in the case of the pixel circuit shown in FIG. 3, brightness data is supplied to the pixel in the form of current.
First, when brightness data is to be written, the scanning line 127 is brought to a selected state (low level in this case), and a current Iw corresponding to the brightness data is passed through the data line 128. The current Iw flows through the TFT 124 to the TFT 125. In this case, let Vgs be a gate-to-source voltage occurring in the TFT 125. Because of a short circuit between the gate and drain of the TFT 125, the TFT 125 operates in a saturation region.
Thus, according to a well-known equation of a MOS transistor, the following holds:
Iw=xcexc1Cox1W1/L1/2(Vgsxe2x88x92Vth1)2xe2x80x83xe2x80x83(1) 
In the equation (1), Vth1 is the threshold value of the TFT 125; xcexc1 is carrier mobility of the TFT 125; Cox1 is gate capacitance per unit area of the TFT 125; W1 is channel width of the TFT 125; and L1 is channel length of the TFT 125.
Then, letting Idrv be a current flowing through the OLED 121, the current value of the current Idrv is controlled by the TFT 122 connected in series with the OLED 121. In the pixel circuit shown in FIG. 3, a gate-to-source voltage of the TFT 122 coincides with the Vgs in the equation (1), and hence, assuming that the TFT 122 operates in a saturation region,
Idrv=xcexc2Cox2W2/L2/2(Vgsxe2x88x92Vth2)2xe2x80x83xe2x80x83(2) 
Incidentally, a condition for operation of a MOS transistor in a saturation region is generally known to be:
|Vds| greater than |Vgsxe2x88x92Vt|xe2x80x83xe2x80x83(3) 
The meanings of the parameters in the equation (2) and the equation (3) are the same as in the equation (1). Since the TFT 125 and the TFT 122 are formed adjacent to each other within a small pixel, it may be considered that actually xcexc1=xcexc2, Cox1=Cox2, and Vth1=Vth2. Then, the following is readily derived from the equation (1) and the equation (2):
Idrv/Iw=(W2/W1)/(L2/L1)xe2x80x83xe2x80x83(4) 
Specifically, even when the values themselves of the carrier mobility xcexc, the gate capacitance Cox per unit area, and the threshold value Vth vary within a panel surface or from panel to panel, the current Idrv flowing through the OLED 121 is in exact proportion to the writing current Iw, and consequently luminous brightness of the OLED 121 can be controlled accurately. In particular, when a design is made such that W2=W1 and L2=L1, for example, Idrv/Iw=1, that is, the writing current Iw and the current Idrv flowing through the OLED 121 are of the same value irrespective of variations in the TFT characteristics.
FIG. 4 is a diagram showing another circuit example of a current writing type pixel circuit. The pixel circuit according to the present circuit example is in opposite relation in terms of a transistor conduction type (N channel/P channel) from the pixel circuit according to the circuit example shown in FIG. 3. Specifically, the N-channel TFTs 122 and 125 in FIG. 3 are replaced with P-channel TFTs 132 and 135, and the P-channel TFTs 124 and 126 in FIG. 3 are replaced with N-channel TFTs 134 and 136. The direction of current flow and the like are also different. However, operating principles are exactly the same.
An active matrix type organic EL display apparatus can be formed by arranging the above-described current writing type pixel circuits as shown in FIG. 3 and FIG. 4 in a matrix manner. FIG. 5 shows an example of configuration of the active matrix type organic EL display apparatus.
In FIG. 5, scanning lines 142-1 to 142-n are arranged one for each of rows of current writing type pixel circuits 141 corresponding in number with m columnsxc3x97n rows and disposed in a manner of the matrix. The gate of the TFT 124 in FIG. 3 (or the gate of the TFT 134 in FIG. 4) and the gate of the TFT 126 in FIG. 3 (or the gate of the TFT 136 in FIG. 4) are connected in each pixel to the scanning line 142-1 to 142-n. The scanning lines 142-1 to 142-n are sequentially driven by a scanning line driving circuit 143.
Data lines 144-1 to 144-m are arranged one for each of the columns of the pixel circuits 141. One end of each of the data lines 144-1 to 144-m is connected to an output terminal for each column of a current driving type data line driving circuit (current driver CS) 145. The data line driving circuit 145 writes brightness data to each of the pixels through the data lines 144-1 to 144-m. 
When such a circuit supplied with brightness data in the form of a current value, that is, a current writing type pixel circuit as shown in FIG. 3 or FIG. 4 is used as a pixel circuit, there is a problem of difficulty in writing low brightness data. In writing data of low brightness extremely close to black, for example, a very small current extremely close to zero is written. In this case, in the circuit example of FIG. 3, impedance of the TFT 125 becomes high, and it takes a long time for potential of the data line having a high parasitic capacitance to be stabilized. This is also true for internal operation of the data line driving circuit 145 of FIG. 5. Therefore, it is generally difficult to supply a very small current quickly and accurately.
The writing of black data means that the value of the writing current is zero, and the writing of complete black takes an infinite time in theory. More specifically, when high brightness data (greater current), for example, is written in a scanning cycle immediately before the writing of black, the data line 128 in FIG. 3 and the data lines 144-1 to 144-m in FIG. 5 are at a relatively high potential. When black is written in the immediately succeeding scanning cycle, the potential of the data line is lowered as a result of action of the TFT 125 in FIG. 3. Since the gate-to-source voltage Vgs of the TFT 125 is decreased as the potential is lowered, the driving current is decreased and the lowering of the potential is slowed quickly. Then, in theory, after passage of an infinite time, the potential of the data line becomes the threshold value voltage Vth of the TFT 125.
Since a practical writing time is finite (commonly one scanning period or less), the gate-to-source voltage of the TFT 122 in FIG. 3 is higher than the threshold value voltage Vth of the TFT 125 at the end of the writing. As described earlier, since the TFT 122 is disposed adjacent to the TFT 125, the threshold value voltage of the TFT 122 is substantially Vth. Therefore, the gate-to-source voltage of the TFT 122 being higher than the threshold value voltage Vth means that the TFT 122 is not completely cut off.
A characteristic (A) in FIG. 6 shows this situation. As a phenomenon, a pixel to which black was to be written actually emits weak light (this phenomenon will hereinafter be described also as xe2x80x9cblack floatingxe2x80x9d). One great advantage of the organic EL display which advantage is not possessed by the liquid crystal display is high contrast ratio. The high contrast ratio results from the capability to display complete black by not passing a current through the light emitting device. However, even slight black floating significantly compromises the contrast ratio of an image, and this represents a problem that cannot be ignored.
In order to solve this problem, the present inventor has also proposed in the above-mentioned patent application (see International Publication Number WO01/06484) a technique for enabling high-contrast image display by providing a leak device (which may hereinafter be referred to as a current bias device or current bias circuit) for each data line. FIG. 7 shows an example of the circuit configuration. An N-channel TFT 129 connected between a data line 128 and a ground in FIG. 7 is the leak device. In a simplest case, a fixed potential is supplied as a gate potential Vg of the TFT 129.
The TFT 129 feeds a bias current Ib in a direction of canceling a driving current Id from a data line driving circuit (data line driving circuit 145 in FIG. 5). Therefore, a rate at which the potential of the data line is lowered at the time of writing black as described above is fast, and in particular, the potential of the data line becoming lower than the threshold value voltage vth in a finite time means the capability of complete black writing. Thus, provision of the leak device for each data line enables high-contrast image display. A characteristic (B) in FIG. 6 shows this situation.
However, the conventional technique of providing the leak device for each data line has the following problems. As shown in FIG. 7, it is practical to use a TFT as the leak device (current bias device). As described at the beginning, however, the TFT has great variations in characteristics, and thus the bias current Ib tends to be varied. A real writing current Iw flowing to the pixel in FIG. 7 at the time of writing brightness data is a result of subtraction of the bias current Ib from the current Id driven by the data line driving circuit, so that brightness of the light emitting device is varied among data lines and actually appears as variations in a form of streaks (streak variations) of a display image.
The streak variations appear as a noticeable problem particularly as the current value of the bias current Ib is set higher. It has therefore been impossible to set the bias current Ib to a high current value. Incidentally, while a simple resistive component may be used as the current bias device, it is generally difficult to provide an appropriate resistance value with good accuracy and in a small area, and thus the resistive component is basically no different from the TFT in that it is difficult to control variations.
The present invention has been made in view of the above problems, and it is accordingly an object of the present invention to provide an active matrix type display apparatus, an active matrix type organic EL display apparatus, and driving methods thereof that are capable of high-quality display of black and low brightness gradation without variations of a display image and capable of image display without variations in brightness when a current writing type pixel circuit is used.
In order to achieve the above object, according to the present invention, there is provided an active matrix type display apparatus comprising: a pixel unit formed by arranging pixel circuits in a matrix manner, the pixel circuits each having an electrooptic device that changes brightness thereof according to a current flowing therein; a data line driving circuit for supplying a writing current of a magnitude corresponding to brightness to each of the pixel circuits via a data line and thereby writing brightness data; and a current driving circuit provided for each data line for feeding the data line with a driving current in a direction of canceling the writing current. The current driving circuit corresponds to current bias circuits in embodiments below. The current driving circuit includes: a converting unit supplied with information of a value of the driving current to be fed in a form of a current, for converting the supplied current into a form of a voltage; a retaining unit for retaining the voltage obtained by the conversion by the converting unit; and a driving unit for converting the voltage retained by the retaining unit into a current, and feeding the data line with the current as the driving current.
In the thus formed active matrix type display apparatus or the active matrix type organic EL display apparatus using an organic EL device as the electrooptic device, when first supplied with information of a driving current value in a form of a current during a period when no data is written to pixels, the current driving circuit converts the current into a form of a voltage and retains the voltage. Then, when data is written to the pixels, the current driving circuit converts the retained voltage into a current and feeds the data line with the current as the driving current in the direction of canceling the writing current, thus using the current as a bias current. In this case, the constant driving current based on the information of the driving current value flows through the data line, and therefore the bias current is not varied among data lines.